Where the topography of a sample surface is investigated by a scanning probe microscope such as a noncontact atomic force microscope (NC-AFM), it is necessary to maintain constant the space between the sample surface and the probe tip. Therefore, this space must be constantly detected. One method of accomplishing this is to utilize changes in the attractive atomic force (i.e., force gradient) exerted between the sample surface and the probe tip. That is, the attractive force increases as the space decreases.
FIG. 6 illustrates a noncontact atomic force microscope (AFM) This microscope has a scanner support portion 01 that supports an XY scanner 02 moving within the XY plane and a Z scanner 03 moving quite small distances along the Z-axis, or up and down. A sample 04 is placed on the top end surface of the Z scanner 03. An XY-scanning signal generator 06 and a Z scanner drive circuit 07 supply a scanning signal and a driving signal, respectively, to the XY scanner 02 and the Z scanner 03, respectively.
A cantilever 08 made of a resilient material is positioned above the sample 04. One end of the cantilever 08 is fixed to piezoelectric plates 09 that apply vibrations to the cantilever. A probe 012 is mounted to the front end of the cantilever 08 such that the tip of the probe 012 faces the sample 04. The piezoelectric plates 09 are made of bimorph and act to vibrate the cantilever 08.
Heretofore, some methods have been available to detect the force gradient in the noncontact atomic force microscope described above.
(a) Slope Detection Method
FIG. 7 illustrates the slope detection method. The cantilever 08 is forcedly vibrated at a frequency of .omega..sub.d. The space S (FIG. 6) between the probe 012 on the cantilever 08 and the sample 04 varies. The resonance frequency of the cantilever 08 changes from .omega..sub.0 to .omega..sub.0 '. At this time, the amplitude of the cantilever varies. The result is shown in the graph of FIG. 7. Where the space S is reduced while the atomic force is exerted between the probe 012 and the sample 04, the resonance frequency of the cantilever 08 drops.
As can be seen from the graph of FIG. 7, where the resonance frequency .omega..sub.0 of the cantilever 08 is moved away from the fixed frequency .omega..sub.d and reaches .omega..sub.0 ', the amplitude of the cantilever 08 forcedly vibrated at .omega..sub.d decreases from A.sub.0 to A.sub.0 '. Accordingly, the changes in the space S can be detected by detecting the increases in the amplitude .DELTA.A. In this way, the slope detection method is to detect changes in the space S between the probe 012 and the sample 04 by detecting the decreases in the amplitude .DELTA.A.
(b) FM Detection Method
In FIG. 6, if the space S between the probe 012 of the cantilever 08 and the sample 04 varies, the resonance frequency of the cantilever 08 changes. That is, where the space S decreases within the range in which an atomic force is exerted between the probe 012 and the sample 04, the resonance frequency of the cantilever 08 drops. Therefore, changes in the space S can be detected by vibrating the cantilever 08 at its resonance frequency at all times and detecting variations in the vibration frequency of the cantilever 08. In this method, the Q value of the mechanical vibration of the cantilever 08 becomes very large in a vacuum. Therefore, it is considered that the FM detection method is more appropriate than the slope detection method that is generally used under atmospheric pressure.
In the above-described FM detection method, the cantilever 08 is vibrated always at its resonance frequency, and changes in the vibrational frequency of the cantilever are detected. Thus, changes in the space S are detected. Therefore, the degree of stability of the oscillating system containing the cantilever 08 greatly affects the performance of the instrument. That is, the ability to supply a stable oscillating signal to the cantilever-vibrating device dominates the performance of the instrument. The following techniques related to this kind of cantilever-vibrating device are known.
FIG. 8 illustrates a noncontact atomic force microscope (AFM) equipped with a cantilever-vibration device making use of the prior art FM detection method. It is to be noted that like components, or 01-012, are indicated by like reference numerals in both FIGS. 6 and 8. A laser 013 directs laser light L onto the cantilever 08. The laser light reflected by the cantilever 08 reaches a photodetector 014, which detects the incident light. This photodetector 014 consists of two discrete photodiodes. The laser light L reflected from the top surface of the cantilever 08 oscillates across the boundary between the two photodiodes. The difference between the output signals from the two discrete photodiodes is a sine wave corresponding to the vibrations of the cantilever 08. The oscillation frequency of the cantilever 08 is detected from this sine wave.
The obtained oscillation signal is fed to an amplifier 017 whose gain is adjusted by an AGC (automatic gain control) circuit 016. This AGC circuit 016 controls the gain of the amplifier 017 in such a way that the amplitude of the output signal from the photodetector 014 is kept constant. The output signal from the amplifier 017 is supplied to a band-pass filter 018, which extracts only frequencies close to the resonance frequency of the cantilever 08. The phase of the output signal from the band-pass filter 018 is adjusted by a phase-adjusting circuit 019. The output signal from the phase-adjusting circuit 019 is supplied as a driving signal to the vibration-applying piezoelectric plates 09. As a result, a self-oscillating positive feedback loop is formed. Consequently, the cantilever 08 is vibrated at its resonance frequency.
The output signal from the photodetector 014 is converted into a voltage signal by a frequency-to-voltage converter circuit 021. The voltage signal Vfv from the converter circuit 021 is sent to a reference voltage comparator 022. This comparator 022 produces the difference between the voltage signal Vfv and a reference voltage signal VfvO and sends a signal to the Z scanner drive circuit 07 via a low-pass filter 023 so that the difference (Vfv-VfvO) becomes zero. The reference voltage signal VfvO is a voltage corresponding to the preset-space between the probe 012 and the sample 04.
The differential signal extracted via the low-pass filter 023 and a scanning signal from the XY scanning signal generator 06 are supplied to an image-creating circuit 024, which in turn creates a topographic image of the surface of the sample 04.
The sample 04 is moved toward the probe 012 while the cantilever 08 is oscillating with a given amplitude, until an atomic force is exerted between the sample 04 and the probe 012. Then, the space is maintained constant. The surface of the sample 04 is scanned within the XY plane in two dimensions by the XY scanner 02. As the distance between the sample 04 and the probe 012 decreases, the resonance frequency of the cantilever 08 decreases by the effect of the atomic force acting on the probe 012. The cantilever 08 vibrates at reduced frequencies. As the distance between the sample 04 and the probe 012 increases, the cantilever vibrates with increasing frequency. At distances where the atomic force can be neglected, the vibration frequency is coincident with the resonance frequency of the cantilever 08 that is intrinsic to the cantilever.
For example, if the surface of the sample 04 has a convex portion, and if the cantilever 08 vibrates at decreasing frequency as the distance between the probe 012 and the sample 04 decreases as a result of the two-dimensional scan made by the XY scanner 02, the voltage signal Vfv drops, thus increasing the differential signal. The Z scanner 03 immediately lowers the sample 04 so that the distance to the probe 012 increases, thus providing feedback control. Therefore, the distance between the probe 012 and the sample 04 is held at a given value determined by the reference voltage VfvO. Since this control is constantly provided, the feedback signal (differential signal) supplied to the Z scanner drive circuit 07 corresponds to the topography of the sample surface. This feedback signal is accepted as an image signal into the image-creating circuit 024 in relation to the two-dimensional scan made by the XY scanner 02. An image is displayed according to the accepted image signal. In this way, a topographic image of the surface of the sample 04 owing to the atomic force can be displayed.
The prior art technique has the following problems. Generally, the bimorph forming the vibration-applying piezoelectric plates 09 has a high degree of sensitivity. That is, these piezoelectric plates are displaced by large amounts when a unit voltage is applied. It is assumed that the cantilever 08 has a spring constant of approximately 40 N/m and a resonance frequency of about 300 kHz. If this cantilever 08 is vibrated at its resonance frequency in an ultrahigh vacuum, the amplitude of vibrations at the tip of the cantilever 08 is tens of thousands times as large as the amplitude of the applied vibrations. Accordingly, if vibrations are applied, using piezoelectric plates having a relatively small sensitivity of about 1 nm/V, and if the amplitude of the vibrations at the tip should be set to the order of nanometers, then the voltage applied to the piezoelectric plates is approximately 0.1 mV. This feeble voltage must be controlled within the oscillator circuit. As a result, the oscillation becomes unstable, and variations in the resonance frequency of the body of the cantilever due to the atomic force are detected with decreased sensitivity.